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The thrill of 3D printing is that is bridges the virtual and the actual. Based on manufacturing technologies developed decades ago, the 3D printing process begins with carefully wrought 3D design files and ends with the robotic arm of a 3D printer flying around to fabricate physical objects of plastic or metal. It's the darling of hacker and steampunk communities and the hope of many who'd love to see a boom in small-scale manufacturing.

At the high end, 3D printers aimed at the aerospace market cost a king's ransom and produce solid hunks of titanium. Cheaper, more versatile laser systems fabricate objects out of melted metal or plastic powders. But the real excitement centers on low-cost 3D printers that use a process called fused deposition modeling (FDM), where plastic wires are melted and deposited to form finished products.

FDM machines are essentially very precise hot-glue guns connected to robots. The printer moves its nozzle to coordinates specified by control software to draw a single layer of the object. The nozzle then deposits the next layer on top of the previous shape. The machine repeats this process dozens of times until it creates a fully formed object, ready for use.

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What are people printing? At the low end, mostly goofy stuff -- everyone who has an inexpensive 3D printer has printed their share of emblem coins, keychain dongles, and USB stick covers. They're toys, really. But sooner or later, the magnitude of being able to create just about any object smaller than a breadbox begins to sink in. Then you start seeing screwdrivers, DIY drones, and all manner of more sophisticated goods.

Originally, 3D printers were invented to fill the needs of manufacturers' R&D departments. The traditional cycle of design, machine by hand, and test -- again and again -- was time consuming and expensive. The movement to outsource machining capability to China has simply drawn out the time delay associated with this cycle. By making the manufacture of test pieces automatic and hands-off, 3D printers result in a much more agile prototying process.

Increasingly, 3D printers are being used for full-time manufacturing. 3D printing turns the mass-production paradigm on its head: Instead of high startup costs and low unit costs once you reach high volume, startup costs are low and the incremental cost for each item you make is the same as that of the first one. Generally, the point where mass production becomes cheaper per unit than 3D printing falls between 10,000 and 100,000 units. In fact, 3D printing can be a less expensive solution for a significant segment of the manufacturing market.

Today, there are dozens of companies vying in the personal 3D printer market. This includes such proprietary commercial players as MakerBot, Stratasys, and 3D Systems, as well as open source upstarts such as Ultimaker and the venerable Reprap project, the granddaddy of the open source 3D printing market.